In the above use it is nowadays known to use so called semi-submersible drilling rigs and the type of jack-up drilling rigs as described above. The above drilling rigs differ from each other in such respect, that semi-submersible drilling rigs comprise an underwater part, which supports the actual working deck existing on the sea level.
Such a drilling rig is not supported stationary on the ground at all during a drilling function, that is why the type of drilling rig requires expensive and complicated joint and movement arrangements between the drilling device drilling the ground and the drilling rig, which enable drilling despite the roll of the sea. Both the costs of manufacturing and operating of such type of drilling rigs are multiple, when compared to drilling rigs of jack-up type described above. One crucial advantage of the type of drilling rigs above compared to nowadays drilling rigs of jack-up type is, however, that because of the massive construction of the same they may be transported in most heterogeneous circumstances, even during a relatively high roll of the sea. In addition to that, they may be used at clearly deeper water, than jack-up rigs, which may be used when the depth of water is usually below 150 m.
Presently many problems are involved with a typical drilling rig of jack-up type, though it has certain significant advantages particularly when compared to those drilling rigs of semi-submersible type described above. First of all it is significantly cheaper to manufacture and to use, since, that the drilling rig of jack-up type is attached stationary on the sea bottom during the drilling situation. This is why complicated joint and movement arrangements are not needed, because the drilling unit drilling the ground does not move significantly with respect to the frame of the drilling rig during the drilling situation. However certain problems are involved with the type of nowadays drilling rigs, that is why they are not safe enough in practice and not useable even during usual circumstances that are characteristic to certain operational areas.
First of all the type of drilling rig may not be safely transported, when the height of the waves is more than 2 m. This is due to the flat shape of the bottom of the frame part, whereby the waves, particularly during a so called transito-phase that is, when the frame part is rising above the sea level when the drilling rig is supported on the sea bottom or respectively when it is lowered on the sea level, when the drilling rig is detached from the sea bottom, hitting the same may cause damages to the frame part and to the devices and equipment placed inside the same. The flatness of the bottom is based on its part on that, that the type of drilling rigs are transported to their operating sites either being towed by towing ships or more commonly transported on the deck of a transporting ship. On the other hand the type of drilling rigs are equipped usually with only three feet to minimize the manufacturing costs, which is the reason why an efficient working deck fitting the same is relatively small. This naturally makes functioning on the drilling rig difficult. In addition, this type of drilling rigs are not considered safe enough in practice since the small size of the drilling rig the residence unit is situated always relatively close by the drilling unit performing the drilling.
This is why despite the advantages of drilling rigs of jack-up type, these advantages rigs may not be exploited with nowadays constructions efficiently enough, not even during usual operating circumstances, that are typical to certain areas, at which the use of the jack-up drilling rig would be otherwise idealistic or clearly more advantageous than the use of a drilling rig of semi-submersible drilling rig described above.
On the other hand for example from U.S. Pat. No. 3,461,828 it is previously known particularly in connection with the semi-submersible drilling rig to blow air below the frame part of the drilling rig to eliminate strokes directed from the sea level towards the bottom of the frame part. The solutions disclosed in this patent are stationary, fixed constructions, which thus may not be particularly adapted in connection with jack-up type drilling rigs, because the bottom of such type of a drilling rig has to be a uniformly flat surface particularly to enable the transportation of the same transferring to the operating site. Though the air-bed being used for example in connection with the US-patent above is meant in principle to eliminate the strokes of the sea level to bottom of the frame part, this matter has a totally different meaning in practice in connection with a drilling rig of jack-up type, because in this connection the type of air-bed arrangement is needed particularly during a so called transito-phase only, that is at that stage, when the frame part rises above the sea level, when the drilling rig is supported on the sea bottom or correspondingly when it lowers on the sea level, when the drilling rig is detached from the sea bottom. This situation is dangerous particularly for a drilling rig of jack-up type, which in practice significantly limits the use of nowadays drilling rigs of jack-up type.